1. Field of the Invention
This invention relates to lighting systems for vision systems.
2. Description of Related Art
The light output of any device is a function of many variables. Some of the variables include the instantaneous drive current, the age of the device, the ambient temperature, whether there is any dirt or residue on the light source, the performance history of the device, etc. Machine vision instrument systems typically locate objects within their field of view using methods which may determine, among other things, the contrast within the region of interest where the objects may be found. To some degree, this determination is significantly affected by the amount of incident light or transmitted light.
Automated video inspection metrology instruments generally have a programming capability that allows an event sequence to be defined by the user. This can be implemented either in a deliberate manner, such as programming, for example, or through a recording mode which progressively learns the instrument sequence. The sequence commands are stored as a part program. The ability to create programs with instructions that perform a sequence of instrument events provides several benefits.
For example, more than one workpiece or instrument sequence can be performed with an assumed level of instrument repeatability. In addition, a plurality of instruments can execute a single program, so that a plurality of inspection operations can be performed simultaneously or at a later time. Additionally, the programming capability provides the ability to archive the operation results. Thus, the testing process can be analyzed and potential trouble spots in the workpiece or breakdowns in the controller can be identified. Without adequate standardization and repeatability, archived programs vary in performance over time and within different instruments of the same model and equipment.
Conventionally, as illustrated in U.S. Pat. No. 5,753,903 to Mahaney, closed-loop control systems are used to ensure that the output light intensity of a light source of a machine vision system was driven to a particular command level. Thus, these conventional closed-loop control systems prevent the output light intensity from drifting from the desired output light intensity due to variations in the instantaneous drive current, the age of the light source, the ambient temperature, or the like.